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January 10, 2012

Scientists Confirm Official Estimate Of Gulf Oil Spill Rate

by editor

Scientists working for the National Oceanic and Atmospheric Administration (NOAA) have determined just how fast gas and oil were leaking into the deep ocean, the surface slick, and into the air during the 2010 Deepwater Horizon oil spill in the Gulf of Mexico.

Using detailed chemical measurements, the researchers estimate an average of 11,130 tons of gas and oil compounds were spilled every day during the disaster. That figure is close to the official average leak rate estimate of 11,350 tons per day originally given.

“This study uses the available chemical data to give a better understanding of what went where, and why,” said Thomas Ryerson, PhD, a NOAA chemist and lead author of the study. “The surface and subsurface measurements and analysis provided by our university colleagues were key to this unprecedented approach to understanding an oil spill.”

To make their estimates, the research team did not use any original data from the spill, such as video analysis, pipe diameter and fluid flow calculations. “We analyzed a completely separate set of chemical measurements, which independently led us to a very similar leak estimate,” said Ryerson.

The analysis, published January 9 in the online edition of the journal Proceedings of the National Academy of Sciences (PNAS), follows on another NOAA-led study published last year, in which Ryerson and his colleagues estimated a lower limit to the Deepwater Horizon leak rate based on two days of airborne analysis collected during the spill and the chemical composition of the reservoir gas and oil determined before the spill.

The new analysis adds data from many other sources, including subsurface and surface samples taken during six weeks of the spill and including a direct measure of the composition of the gas and oil leaking into the Gulf.

The team found that leaking gas and oil quickly separated into three major pools: an underwater plume of droplets deep below the surface, a visible slick on the surface, and an airborne plume of evaporating chemicals. Each pool had a very different chemical composition.

The team found the underwater plume, 3,300 to 4,300 feet below the surface, was enhanced in gases known to dissolve readily in water. The surface slick was dominated by the heaviest and stickiest components, which neither dissolved in seawater nor evaporated into the air. The airborne plume contained a wide mixture of intermediate-weight components of the spilled gas and oil.

The deep water plume represented about 36 percent of the gas and oil leaked, while 15 percent was represented by the surface slick, and 7 percent for the airborne plume. About 17 percent of the gas and oil was recovered directly to the surface through a marine riser. The remaining 25 percent of the total is not directly accounted for by NOAA´s data.

Experts hope the information about the transport and fate of different components of the spilled gas and oil mixture could help resource managers and other trying to understand environmental exposure levels.

The new chemistry-based estimate of 11,130 tons per day has an estimated range of 8,900 to 13,300 tons per day. By comparison, the official estimated range was 10,000 to 12,700 tons per day.

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Image Caption: A view of the oil source as seen during an overflight on May 20, 2010. Credit: NOAA